The technical requirements for a switching circuit used to provide a direct line connection between a transmission line and a telephone, teleprinter or like communication instrument are quite stringent, particularly in public utility systems. Thus, a direct line connect circuit of this kind may be required to afford an "off" resistance of at least 20 megohms at 200 volts D.C., allowing for a leakage current of no more than 10 microamperes. This requirement must be met over a relatively broad range of temperatures and other varying operating conditions.
Electromechanical relays can be utilized for direct line connect switching circuits in communication systems. For many applications, however, relays are too slow, too large, and too costly. In addition, they are likely to present service problems if the operating environment for the communication instrument with which they are employed includes excessive quantities of dust or other pollutants in the air.
Mercury-wetted contact relays are also employed in direct line connect switching circuits. These devices afford substantial advantages in size and speed of operation as compared with electromechanical relays. The mercury relays, however, will not tolerate changes in orientation of the apparatus. Thus, a communication instrument constructed for horizontal orientation cannot be operated in a substantially vertical orientation if this kind of line connect switch is employed. Furthermore, mercury-wetted contact relays are sensitive to external magnetic fields, and hence may be subject to spurious operation if mounted in close proximity to a power transformer or other device in the communication station that produces an appreciable magnetic field.
Conventional solid state switching circuits, on the other hand, do not ordinarily meet the operational requirements of direct line connect communication system service. They often do not afford sufficient isolation from the transmission line. In addition, the off resistance requirements mentioned above are difficult to meet with this kind of switch.